انجام نشدن عمل undo & redo در برنامه گرافیکی

مشکل این تکه کد چیه که عمل undo & redo انجام نمیشه . (کد های زیر قسمتی از یک برنامست که اینجا کپی کردم )

سپاس

 private Stack<Image> _undostack = new Stack<Image>();
        private Stack<Image> _redostack = new Stack<Image>();

 public void undo()
        {
           
                if (_undostack.Count > 0)
                {
                    _redostack.Push(_undostack.Pop());
                    pictureBox1.Image = _redostack.Peek();
                    pictureBox1.Refresh();
                }

            

        }
        public void redo()
        {
       
                if (_redostack.Count > 0)
                {
                    _undostack.Push(_redostack.Pop());
                    pictureBox1.Image = _undostack.Peek();
                    pictureBox1.Refresh();

                }
            
        }


  private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {

               Graphics g = Graphics.FromImage(bmp);
               Pen p = new Pen(currentcolor, drawingsize);
               SolidBrush brush = new SolidBrush(Color.White);
               if (act == "pen")
               {
                   if (e.Button == MouseButtons.Left)
                   {
                       g.DrawLine(p, x, y, e.X, e.Y);
                       x = e.X;
                       y = e.Y;

                   }
                   x = e.X;
                   y = e.Y;
                   pictureBox1.Image = bmp;
                   _undostack.Push(pictureBox1.Image);
               }

 private void undoToolStripMenuItem_Click(object sender, EventArgs e)
        {
            undo();
            redoToolStripMenuItem.Enabled = true;
            undoToolStripMenuItem.Enabled = false;         
        }


        private void redoToolStripMenuItem_Click(object sender, EventArgs e)
        {
            redo();
            redoToolStripMenuItem.Enabled = false;
            undoToolStripMenuItem.Enabled = true;
            
        }